Spherical coordinate support

[jamesmlane]

Add spherical coordinate methods to orbit.Orbit and cylindrical <--> spherical transformation methods to util.bovy_coords

[codecov]

Codecov Report

Merging #427 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           master     #427    +/-   ##
========================================
  Coverage   99.81%   99.82%            
========================================
  Files         162      166     +4     
  Lines       24947    25173   +226     
========================================
+ Hits        24902    25128   +226     
  Misses         45       45            

Impacted Files	Coverage Δ
galpy/orbit/Orbits.py	99.94% <100.00%> (+<0.01%)	⬆️
galpy/util/bovy_coords.py	100.00% <100.00%> (ø)
galpy/potential/__init__.py	100.00% <0.00%> (ø)
galpy/orbit/integrateFullOrbit.py	100.00% <0.00%> (ø)
galpy/orbit/integratePlanarOrbit.py	100.00% <0.00%> (ø)
galpy/potential/BurkertPotential.py	100.00% <0.00%> (ø)
galpy/potential/SphericalShellPotential.py	100.00% <0.00%> (ø)
galpy/orbit/orbit_c_ext/integrateFullOrbit.c	100.00% <0.00%> (ø)
galpy/orbit/orbit_c_ext/integratePlanarOrbit.c	100.00% <0.00%> (ø)
...lpy/potential/potential_c_ext/SphericalPotential.c	100.00% <0.00%> (ø)
... and 4 more

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[jobovy]

Looks great, just a few very minor comments. I generated the docs and they look fine for me, so perhaps an issue with sphinx on your end? Anyway, docs seem fine.